This application is designed to provide the Principal Investigator with support to continue his development as an independent physician- scientist with sufficient protected time to pursue his research in gene discovery and provide mentorship to trainees. Dr. Gelb is an Associate Professor of Pediatrics and Human Genetics at the Mount Sinai School of Medicine. Clinically, he is a board certified pediatric cardiologist with expertise in the genetics of cardiovascular diseases. he directs a molecular genetics laboratory which investigates Mendelian and complex traits. The principal focus of his laboratory has been the study of pycnodysostosis (Pycno), an osteosclerotic bone dysplasia. Using a positional cloning approach, Dr. Gelb first established the Pycno critical region at chromosome 1q21 by linkage analysis, and then identified the disease gene as achtepsin K, a lysosomal cysteine protease. He has completed linkage studies for other Mendelian traits, including another osteosclerotic bone disorder, Kenny-Caffey syndrome (KCS). In the proposed research, the Principal Investigator intends to elucidate the critical role of Cathepsin K mutations will identified and then over expressed in Pichia pastoris. Mutant enzyme proteins will be characterized biochemically and the results compared OT the structural perturbations predicted by molecular modeling. In order to facilitate genotype-phenotype correlations as well as to better characterize this disorder, abnormalities in bone metabolism will be assessed in Pycnopatients using non-invasive markers, and the degree of cortical bone thickening and medullary narrowing documented by MRI. To understand the growth hormone (GH) deficiency associated with pycno, provocative testing of GH secretion and other pituitary functions will be pursued , and the anatomic status of the pituitary gland will be evaluated by MRI. Pycno osteoclast-like cells, generated in vitro from peripheral blood mononuclear cells, will be assessed functionally using a bone pit formation assay. Generalize inhibitors of cysteine cathepsins and metalloproteases will be used to document the nature of the residual proteolytic activity in Pycno. Transcript levels of known cysteine cathepsins will be assessed to detect upregulation of related proteases. Novel cysteine cathepsins with potential role in bone resorption will be sought from cathepsin K-deficient osteoclasts. Finally, studies will be initiated to identify the KCS gene which appears to have a central role in bone metabolism and calcium homeostasis. The KCS critical region will be refined by developing novel polymorphic markers and recruiting additional KCS families. Positional candidate genes, either known or from ESTs, will be scanned for mutations. Thereafter novel genes will be cloned from genomic clones spanning the KCS critical region. The Principal Investigator~s research career development will be supported with protected research time, dedicated laboratory space, Departmental and Institutational core facilities, as well as by the superb intellectual environment at Mount Sinai. he also will serve as a mentor for high quality M.D. and M.D./Ph.D. trainees who will be supported by NIH-funded training programs in his Departments and institution.